A spark plug is mounted to a cylinder head of an internal combustion engine by screwing into a plug mounting hole of the cylinder head. For this reason, a thread portion is formed on an outer circumferential surface of a metal shell of the spark plug. It is conceivable, at the formation of the thread portion, to adjust the start position of thread forming operation in such a manner that, in a state where the spark plug is mounted to the internal combustion engine, a ground electrode of the spark plug is oriented to a predetermined direction within a combustion chamber of the internal combustion engine.
For example, Japanese Laid-Open Patent Publication No. 2001-284015 (abbreviated as “JP2001-284015A”) discloses a manufacturing apparatus of a spark plug, which forms a thread portion by rolling a part of workpiece to be processed into a metal shell of the spark plug with the use of rolling dies.